Electro-pneumatic converters are known wherein a 4 to 20 milliamp electrical signal is used to actuate a solenoid-like motor. The 4 to 20 milliamp electrical signal causes a proportionate displacement in the core of the solenoid-like motor which displacement is used to control an associated valve producing a pressure change proportional to the motion of the core. An example of such a device may be found in U.S. Pat. No. 3,334,642 issued Aug. 8, 1967 to P. G. Borthwick.
There are certain disadvantages to such known electro-pneumatic control systems. Firstly, they are unable to hold positions on loss of power. Should power be removed from the coil, the core moves back to a position where it is in equilibrium. This causes the pneumatic output signal to go off scale, resulting in the movement of control devices to the fully-opened or fully-closed position which may be catastrophic in certain control systems. Secondly, such known electro-pneumatic converters are vibration sensitive. Since the cores are suspended from springs which act as range and zero limiters, vibration of the core causes a variation in the pneumatic output signal.